The polymerization of monomers, particularly of styrene during processing is a matter of concern, because it causes formation of unwanted polymers and results in loss of yield of end product and makes the process un-economical.
In the art use of inhibitors and retarders, and combination thereof to overcome problem of polymerization of styrene has been reported.
The problem of using the inhibitors alone is that these are to be added continuously or at regular interval, because once they are consumed, the polymerization will re-start.
The problem of using the retarders is that these are not very effective to reduce polymerization of styrene to a level of substantial inhibition or to the acceptable level of inhibition.
The prior art discloses use of quinone methide (QM) as polymerization inhibitor. However, the inventor has found [refer to examples] that main problem of using the quinone methide is that it has to be used in higher amounts to achieve acceptable level of inhibition, and such higher amount not only result in increase of cost of process, but also result in formation of undesired products due to unstable nature of quinone methide.
The prior art also proposes quinone methide based composition comprising quinone methide and 4HT (4 hydroxy tempo 2,2,6,6-tetramethyl-,1-oxide) as styrene polymerization inhibitor. However, the inventor has found [refer to examples] that main problem of using this known composition of quinone methide is that even at higher amounts the problem of polymerization is not resolved to acceptable level.
The prior art [U.S. Pat. No. 7,651,635] discloses use of combination of an inhibitor and a retarder, wherein the inhibitor is polymerization inhibitor consisting of alkylhydroxylamine and retarder is 7-substituted quinone methide. The main problem of this composition is that it employs inhibitor, which are consumed continuously and get gradually depleted, and hence, the inhibitor has to be added continuously or intermittently or at least it has to be ensured that an appropriate amount of the inhibitor is maintained in the system [Col. 4, lines 7-13 of U.S. Pat. No. '635].
The prior art D1 (US 2004/034247 A1) discloses additive composition for control and inhibition of polymerization of styrene comprising quinone methide 4-benzylidene,2,6-di-tert-butyl-cyclohexa-2,5-dienone (QM), hydroxylamine and catechol, which is 4-tert-butyl catechol (TBC) (re abstract, paragraphs [0009], [0019], [0024], Examples, Table 1, claim 1, claim 10 of D1).
The prior art D2 (US 2006/020089 A1) discloses use of phenylenediamine in combination with QM (re its Example 3, paragraph no. [0033]).
The prior art D3 (U.S. Pat. No. 6,342,648 B1) discloses oxidised amines, but it neither discloses nor suggests the combination thereof with methides.
In accordance with present invention, the “acceptable level of inhibition of polymerization of styrene” is inhibition of polymerization of at least 98.5%, preferably at least 99% of styrene, that is only up to 1.5%, especially up to 1% of styrene polymerizes.